basic/strv: fix strv_join for first empty argument

Empty strings were ignored in strv_join, but only if they were at the beginning
of the string. Empty strings after at least one non-empty item were treated
normally.

Previously:
{"x"} → "x"
{"x", ""} → "x"
{"x", "", ""} → "x::"
{""} → ""
{"", ""} → ""
{"", "", ""} → ""
{"", "x"} → "x"
{"", "x", ""} → "x:"
Now:
{"x"} → "x"
{"x", ""} → "x"
{"x", "", ""} → "x::"
{""} → ""
{"", ""} → ":"
{"", "", ""} → "::"
{"", "x"} → ":x"
{"", "x", ""} → "❌"

src/basic/strv.c

@@ -371,7 +371,7 @@ char *strv_join(char **l, const char *separator) {
 
         n = 0;
         STRV_FOREACH(s, l) {
-                if (n != 0)
+                if (s != l)
                         n += k;
                 n += strlen(*s);
         }
@@ -382,7 +382,7 @@ char *strv_join(char **l, const char *separator) {
 
         e = r;
         STRV_FOREACH(s, l) {
-                if (e != r)
+                if (s != l)
                         e = stpcpy(e, separator);
 
                 e = stpcpy(e, *s);